2022 May/June · Paper 11 · Q91 markElements & structure of molecules
2022 May/June · Paper 11 · Q111 markFood tests
2022 May/June · Paper 12 · Q91 markElements & structure of molecules
2022 May/June · Paper 12 · Q111 markFood tests
2022 May/June · Paper 13 · Q91 markElements & structure of molecules
2022 May/June · Paper 13 · Q111 markFood tests
2022 May/June · Paper 21 · Q71 markElements & structure of molecules
2022 May/June · Paper 21 · Q81 markDNA structure & base pairing
2022 May/June · Paper 22 · Q71 markElements & structure of molecules
2022 May/June · Paper 22 · Q81 markDNA structure & base pairing
2022 May/June · Paper 23 · Q71 markElements & structure of molecules
2022 May/June · Paper 23 · Q81 markDNA structure & base pairing
2022 May/June · Paper 32 · Q7 (c)6 marksElements & structure of molecules
2022 May/June · Paper 33 · Q7 (c(ii))1 markElements & structure of molecules
2022 May/June · Paper 43 · Q2 (a(i))2 marksElements & structure of molecules
2022 May/June · Paper 61 · Q1 (b)2 marksFood tests
2022 May/June · Paper 62 · Q2 (d)2 marksFood tests
2022 May/June · Paper 63 · Q2 (b(iii))3 marksFood tests
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or see the official Cambridge past-papers page; for other papers, ask your
school. “Paper 42” means Paper 4, variant 2.
Practice questions
Original questions written by HomiLabs in the style of the exam, with answers and mark schemes. They are not
Cambridge past paper questions and have not been reviewed by Cambridge. Try each one before opening the answer.
Q1Food tests · Core · Paper 4 style · 4 marks
(a)A student wants to test four separate food samples for (i) starch, (ii) reducing sugar, (iii) protein and (iv) fat. For each nutrient, name the reagent used and state the colour seen when the nutrient is present.[4]
1 mark per nutrient for the correct reagent AND the correct positive colour (4 × B1). Accept 'cloudy white' for the emulsion test; do not accept 'pink' for biuret.
Q2Food tests · Core · Paper 6 style · 5 marks
A student in Multan squeezes a ripe mango to obtain some juice. She wants to find out whether the juice contains reducing sugar.
(a)Describe how the student should carry out the Benedict's test on a sample of the juice.[3]
(b)The juice turned from blue to brick-red. State what this shows.[1]
(c)State one safety precaution the student should take during this test.[1]
Show answer
(a) Put about 2 cm³ of juice in a test-tube, add an equal volume of Benedict's solution, heat in a hot water-bath (about 80 °C) for a few minutes and record the colour.
B1 add Benedict's solution to the juice; B1 heat in a (hot / boiling) water-bath; B1 heat for a stated time (e.g. 2–5 minutes) / observe colour change. Do not credit 'heat over a Bunsen flame' alone.
(b) The juice contains a reducing sugar (a large amount).
B1 reducing sugar present; accept 'high concentration of reducing sugar'.
(c) Wear eye protection (goggles) / point the test-tube away from people / use a test-tube holder.
B1 any one sensible precaution linked to heating.
Q3Food tests · Core · Paper 2 style · 1 mark
(a)[1]
Show answer
(a) D brick-red
Brick-red shows the highest concentration of reducing sugar; blue means none. Green and yellow show smaller amounts.
Q4Food tests · Core · Paper 6 style · 4 marks
A student places a few drops of iodine solution on each of two samples on a white tile: the cloudy water left after boiling rice, and a solution of table sugar.
(a)State the colour the student would see if starch is present.[1]
(b)The student adds iodine solution to the sugar solution. State the colour seen and what this shows.[2]
(c)Suggest why the student uses a separate clean dropper for each sample.[1]
Show answer
(a) Blue-black.
B1 blue-black; accept 'black' or 'dark blue'.
(b) Stays orange-brown (yellow-brown); starch is absent.
B1 orange / brown / yellow-brown colour; B1 no starch.
(c) To avoid contaminating one sample with another (which could give a false result).
B1 idea of contamination / cross-contamination.
Q5Food tests · Core · Paper 4 style · 5 marks
A breakfast in Lahore consists of a paratha fried in ghee, a boiled egg and a glass of lassi.
(a)Describe how the emulsion test could be used to show that the paratha contains fat.[3]
(b)Name the reagent used to show that the egg contains protein and state the positive result.[2]
Show answer
(a) Crush / cut the paratha and shake it with ethanol; pour the ethanol (liquid) into a tube of water; a milky white emulsion shows fat.
B1 shake (crushed) food with ethanol; B1 add the ethanol to water (or add water); B1 cloudy / milky white = fat present.
(b) Biuret solution; the colour changes from blue to purple.
B1 biuret; B1 purple / violet / lilac.
Q6Food tests · Core · Paper 6 style · 4 marks
A student mixes some grated cheese with water to make a cheese solution for a protein test.
(a)Describe how the biuret test is carried out on the cheese solution.[2]
(b)The student also sets up a tube containing distilled water instead of cheese solution, and adds the same reagents. Explain why.[2]
Show answer
(a) Add (a few drops of) dilute sodium hydroxide and then (dilute) copper(II) sulfate solution — or add biuret solution — and shake; no heating.
B1 add biuret reagent / sodium hydroxide + copper sulfate; B1 shake / mix and observe (no heating needed).
(b) It is a control; it shows the colour of a negative result (blue), so any purple in the test tube must be due to protein in the cheese.
B1 control / comparison; B1 shows the colour without protein / shows the reagent alone does not turn purple.
Q7Food tests · Standard · Paper 6 style · 6 marks
A student heats 2 cm³ of each of five glucose solutions with 2 cm³ of Benedict's solution for 5 minutes in a boiling water-bath. She does the same with 2 cm³ of guava juice.Glucose concentration / % : colour after 5 minutes0 : blue0.5 : green1.0 : yellow2.0 : orange4.0 : brick-redguava juice : yellow-orange
(a)Using the table, estimate the concentration of glucose in the juice.[1]
(b)Name two variables that must be kept the same for every tube.[2]
(c)Suggest one change to the method that would give a more precise estimate.[1]
(d)Explain why the student kept all the tubes in the same water-bath and recorded the colour after the same time.[2]
Show answer
(a) Between 1 % and 2 % (more than 1 %, less than 2 %).
B1 range 1–2 %.
(b) Any two of: volume of solution / volume of Benedict's solution / temperature of water-bath / heating time.
1 mark each for any two controlled variables.
(c) Make more glucose standards at smaller intervals between 1 % and 2 % (e.g. 1.2, 1.4, 1.6, 1.8 %) / use a colorimeter instead of judging colour by eye.
B1 any one valid improvement.
(d) Colour depends on temperature and time of heating; keeping them the same makes it a fair comparison so colour depends only on glucose concentration.
B1 time / temperature affect the colour reached; B1 fair test / only glucose concentration varies.
Q8Food tests · Standard · Paper 4 style · 5 marks
A student wants to show that roasted groundnuts (peanuts) contain fat.
(a)Explain why the test-tube used for the emulsion test must be dry when the crushed nuts are first shaken with ethanol.[2]
(b)Explain why a milky white colour appears when the ethanol is poured into water.[2]
(c)Suggest why the ethanol should not be heated with a Bunsen burner.[1]
Show answer
(a) Fat does not dissolve in water; any water present would form the emulsion too early / stop the fat dissolving in the ethanol, so the result would be unclear.
B1 fat dissolves in ethanol (not in water); B1 water would interfere / give a false or unclear result.
(b) The fat that was dissolved in ethanol comes out of solution because fat is insoluble in water; tiny droplets of fat are suspended (an emulsion), which scatter light and look cloudy.
B1 fat insoluble in water / comes out of solution; B1 forms tiny droplets / emulsion.
(c) Ethanol is flammable.
B1.
Q9Food tests · Standard · Paper 2 style · 1 mark
(a)[1]
Show answer
(a) B protein and fat
Purple biuret = protein; milky emulsion = fat. Iodine orange-brown means no starch (so A and D are wrong) and Benedict's blue means no reducing sugar (so C is wrong).
Q10Food tests · Standard · Paper 4 style · 4 marks
Two shops in Peshawar sell honey. A student wants to compare the amount of reducing sugar in each honey after diluting each with water in the same way.
(a)The Benedict's test is described as semi-quantitative. Explain what this means.[2]
(b)Suggest two ways the results of Benedict's tests on two honey samples could be made more quantitative.[2]
Show answer
(a) It gives an approximate idea of how much reducing sugar is present (by the colour reached), but not an exact concentration.
B1 shows relative amount / range; B1 not an exact value (colours judged by eye).
(b) Any two of: use a colorimeter to measure the colour; filter, dry and weigh the red precipitate; time how long each sample takes to change colour; compare with a range of glucose standards of known concentration.
1 mark each for any two.
Q11Food tests · Standard · Paper 6 style · 6 marks
A student compares the amount of reducing sugar in apple, orange and pomegranate juice using the Benedict's test.
(a)State the independent variable and the dependent variable in this investigation.[2]
(b)State three variables that should be kept constant.[3]
(c)Suggest why each juice should be tested three times.[1]
Show answer
(a) Independent: type of fruit juice. Dependent: colour of the Benedict's solution after heating (amount of reducing sugar).
B1 each.
(b) Any three of: volume of juice; volume of Benedict's solution; temperature of water-bath; time of heating; dilution of juice; size of test-tube.
1 mark each, max 3.
(c) To check the results are repeatable / to identify anomalous results.
B1.
Q12Food tests · Standard · Paper 5 style · 5 marks
A student adds orange juice drop by drop from a dropping pipette to 1 cm³ of blue DCPIP solution, shaking after each drop, and counts the drops needed to decolourise it.
(a)Suggest why it is difficult to decide exactly when the DCPIP has been decolourised by orange juice.[1]
(b)Describe two other sources of error in the method and suggest an improvement for each.[4]
Show answer
(a) The juice is coloured itself (orange), so the end-point colour is hard to judge / the colour fades gradually.
B1.
(b) e.g. size of drops varies → use a burette or graduated syringe; tube not shaken evenly → shake after every drop the same way; judging the colour → compare against a white background / a reference tube of decolourised DCPIP; juice left standing loses vitamin C → test immediately after squeezing.
For each of two errors: B1 error, B1 matching improvement. Generic 'repeat it' alone scores 0.
Q13Food tests · Challenge · Paper 6 style · 6 marks
DCPIP is a blue solution that is decolourised by vitamin C. A student finds the volume of each liquid that must be added, drop by drop, to decolourise 1.0 cm³ of DCPIP.Vitamin C solution, 1.0 mg/cm³ : 2.5 cm³Fresh lemon juice : 4.0 cm³Lemon juice boiled for 10 minutes : 6.4 cm³
(a)Calculate the mass of vitamin C needed to decolourise 1 cm³ of the DCPIP solution.[1]
(b)Calculate the concentration of vitamin C, in mg/cm³, in (i) the fresh lemon juice and (ii) the boiled lemon juice. Give (ii) to 2 significant figures.[2]
(c)Calculate the percentage decrease in vitamin C concentration caused by boiling.[2]
(d)State why a larger volume of boiled juice was needed.[1]
(0.625 − 0.3906) ÷ 0.625 × 100 (M1) = 37.5 % (A1). Accept 37.6 % if the rounded value 0.39 is used.
(d) Boiling destroys (breaks down) some of the vitamin C, so the juice has less vitamin C per cm³.
B1.
Q14Food tests · Challenge · Paper 4 style · 6 marks
A length of Visking tubing (a partially permeable membrane) is filled with a mixture of starch suspension and glucose solution. The tubing is rinsed and placed in a tube of distilled water.
(a)Predict the results of an iodine test and a Benedict's test on the water outside the tubing after 30 minutes. Explain your predictions.[4]
(b)Suggest what this model shows about digestion in the small intestine.[2]
Show answer
(a) Iodine: stays orange-brown — no starch, because starch molecules are too large to pass through the tubing. Benedict's: green/yellow/orange/red — reducing sugar present, because glucose molecules are small enough to diffuse out through the tubing.
B1 iodine negative; B1 starch molecules too large; B1 Benedict's positive; B1 glucose small, diffuses through (partially permeable) tubing.
(b) Large insoluble molecules like starch must be broken down into small soluble molecules (e.g. glucose) before they can be absorbed through the gut wall into the blood.
B1 large molecules cannot be absorbed; B1 must be digested into small soluble molecules first.
Q15Food tests · Challenge · Paper 5 style · 6 marks
(a)Plan an investigation to compare the vitamin C content of freshly squeezed orange juice with orange juice that has been kept in an open glass at room temperature for 24 hours. Include the apparatus, method, variables to control, how results are recorded and how a conclusion is reached.[6]
Show answer
(a) DCPIP titration: same volume of DCPIP; add each juice drop by drop from a burette/syringe until decolourised; record volume; repeat three times; less volume = more vitamin C.
1 mark each, max 6: use DCPIP (blue, decolourised by vitamin C); fixed volume of DCPIP (e.g. 1 cm³) in a test-tube; add juice from burette / graduated pipette / syringe drop by drop, shaking; record volume of juice when blue colour disappears; same oranges / same juice divided into two portions; same temperature / same concentration of DCPIP; repeat at least three times and calculate a mean; a smaller volume of juice needed means more vitamin C; table with headings 'volume of juice / cm³' for each juice; safety: eye protection.
Q16Food tests · Challenge · Paper 6 style · 5 marks
A student times how long it takes for Benedict's solution to show the first colour change (from blue to green) when heated with glucose solutions in a water-bath at 90 °C.Glucose concentration / % : time / s0.2 : 1500.4 : 950.6 : 700.8 : 521.0 : 40
(a)Describe the trend in the results.[1]
(b)A sample of sugar-cane juice, diluted in the same way, took 80 s to show the first colour change. Assuming the change is linear between the two nearest readings, estimate the glucose concentration of the diluted juice. Give your answer to 2 decimal places.[2]
(c)Explain why judging the 'first colour change' is a source of error and suggest how to reduce it.[2]
Show answer
(a) As glucose concentration increases, the time for the first colour change decreases (falling more slowly at higher concentrations).
B1.
(b) 0.52 %
80 s lies between 0.4 % (95 s) and 0.6 % (70 s); 0.4 + (95 − 80)/(95 − 70) × 0.2 (M1) = 0.52 % (A1).
(c) The change is gradual and judged by eye, so it is subjective; compare against a reference tube / white background or use a colorimeter.
B1 subjective / gradual; B1 improvement.
Q17Food tests · Challenge · Paper 4 style · 5 marks
Maize grains are soaked and left to germinate. A student crushes samples on day 0 and day 4 and tests them.Day 0: iodine test blue-black; Benedict's test green.Day 4: iodine test pale blue-black; Benedict's test brick-red.
(a)Describe the changes in the results between day 0 and day 4.[2]
(b)Explain these changes.[3]
Show answer
(a) Starch content decreases (blue-black becomes paler / orange-brown); reducing sugar increases (Benedict's goes from blue/green to orange/brick-red).
B1 starch decreases; B1 reducing sugar increases.
(b) Amylase (enzyme) becomes active in the germinating seed; it breaks down stored starch into maltose / reducing sugars; the sugars are soluble and are used in respiration to release energy for growth of the embryo.
B1 enzyme / amylase; B1 starch → reducing sugar (maltose); B1 sugar used for respiration / energy / growth.
Q18Food tests · Challenge · Paper 2 style · 1 mark
(a)The same volume of DCPIP solution is decolourised by 2 cm³ of juice X but needs 8 cm³ of juice Y. Which statement is correct?[1]
Show answer
(a) B Juice X contains four times the concentration of vitamin C of juice Y.
The same mass of vitamin C is needed each time. X supplies it in 2 cm³ and Y in 8 cm³, so X is 8 ÷ 2 = 4 times more concentrated. A is the common error of assuming a larger volume means more vitamin C.
Q19Elements & structure of molecules · Core · Paper 4 style · 5 marks
(a)State the chemical elements found in (i) carbohydrates, (ii) fats and (iii) proteins.[3]
(b)State the smaller molecules that join to make (i) proteins and (ii) fats.[2]
Show answer
(a) (i) carbon, hydrogen, oxygen; (ii) carbon, hydrogen, oxygen; (iii) carbon, hydrogen, oxygen and nitrogen (some also sulfur).
B1 each; (iii) must include nitrogen.
(b) (i) amino acids; (ii) fatty acids and glycerol.
B1 each; (ii) needs both.
Q20Elements & structure of molecules · Core · Paper 2 style · 1 mark
(a)Which smaller molecules join together to form glycogen?[1]
Show answer
(a) C glucose
Glycogen, like starch and cellulose, is a chain of glucose molecules. Sucrose is a sugar made of two units and is not the building block.
Q21Elements & structure of molecules · Standard · Paper 4 style · 3 marks
Water makes up about 60 % of the mass of an adult human body.
(a)Explain why water is important as a solvent in the human body. Give three examples.[3]
Show answer
(a) Any three of: digested food molecules dissolve in water so they can be absorbed; plasma (mainly water) transports dissolved glucose, ions, hormones and CO₂; urea and other wastes are excreted dissolved in water (urine); enzyme reactions take place in solution in cells.
1 mark each for any three examples linked to dissolving.
Q22Elements & structure of molecules · Standard · Paper 4 style · 4 marks
(a)Starch, glycogen and cellulose are all made from the same small molecule. Name it.[1]
(b)For each of starch, glycogen and cellulose, state whether it is found in plants or animals and give one role.[3]
Show answer
(a) Glucose.
B1.
(b) Starch: plants — energy store. Glycogen: animals (liver, muscles) — energy store. Cellulose: plants — forms cell walls.
B1 each for correct location AND role.
Q23Elements & structure of molecules · Challenge · Paper 4 style · 5 marks
A scientist analyses two substances taken from the cells of a cow.
(a)Substance X contains carbon, hydrogen, oxygen, nitrogen and sulfur. Suggest which group of biological molecules X belongs to. Explain.[2]
(b)Substance Y contains only carbon, hydrogen and oxygen. It is insoluble in water, is stored in liver cells, and when broken down releases glucose. Identify Y and explain why being insoluble is useful for a storage molecule.[3]
Show answer
(a) Protein; only proteins (of these groups) contain nitrogen (and sometimes sulfur).
B1 protein; B1 reason about N / S.
(b) Glycogen; insoluble so it does not dissolve in cytoplasm / does not affect water movement by osmosis / cannot diffuse out of the cell.
B1 glycogen; B1 does not affect osmosis / water potential; B1 stays in the cell / does not move out.
Q24Elements & structure of molecules · Challenge · Paper 4 style · 4 marks
(a)A student says 'proteins and carbohydrates contain exactly the same elements'. Explain why the student is wrong.[1]
(b)Explain how the order of amino acids in a protein can affect the way an enzyme works.[3]
Show answer
(a) Proteins also contain nitrogen (and sometimes sulfur); carbohydrates contain only carbon, hydrogen and oxygen.
B1.
(b) The sequence of amino acids determines how the chain folds and so the shape of the protein; the shape of the active site depends on this; a different order gives a different-shaped active site, so the substrate may not fit (not complementary) and the enzyme does not work.
B1 sequence determines folding / shape; B1 active site shape; B1 substrate fits / complementary — changed order means it may not fit.
Q25DNA structure & base pairing · Core · Paper 4 style · 4 marks
(a)Describe the structure of a DNA molecule.[3]
(b)One strand of a short piece of DNA has the base sequence A T G C C T A G. Write the sequence of bases on the complementary strand.[1]
Show answer
(a) Two strands coiled together to form a double helix; each strand contains chemicals called bases; bases on the two strands are joined in pairs (A with T, C with G).
B1 two strands; B1 double helix; B1 bases pair / bonds between bases hold strands together; accept named pairs.
(b) T A C G G A T C
B1 all correct.
Q26DNA structure & base pairing · Core · Paper 2 style · 1 mark
(a)Which pairs of bases join together in a DNA molecule?[1]
Show answer
(a) C A with T, and C with G
Adenine pairs with thymine and cytosine pairs with guanine. B pairs bases wrongly.
Q27DNA structure & base pairing · Standard · Paper 4 style · 4 marks
A sample of double-stranded DNA from the cells of a goat is analysed. 22 % of the bases are adenine (A).
(a)State the percentage of thymine in this DNA. Explain your answer.[2]
(b)Calculate the percentage of cytosine in this DNA.[2]
Show answer
(a) 22 %; A always pairs with T, so there is the same amount of each.
B1 22 %; B1 A pairs with T.
(b) 28 %
100 − (22 + 22) = 56 (M1); 56 ÷ 2 = 28 % (A1).
Q28DNA structure & base pairing · Standard · Paper 4 style · 4 marks
One strand of a short piece of DNA has the sequence A T T G C A G C T A.
(a)Write the base sequence of the complementary strand.[1]
(b)For the double-stranded piece, state the total number of adenine bases and the total number of guanine bases.[2]
(c)Calculate the percentage of guanine in the double-stranded piece.[1]
Show answer
(a) T A A C G T C G A T
B1.
(b) A = 6; G = 4
B1 each.
(c) 20 %
4 ÷ 20 × 100 = 20 % (B1).
Q29DNA structure & base pairing · Challenge · Paper 4 style · 4 marks
A section of double-stranded DNA from a wheat plant contains 600 base pairs. 180 of the bases in this section are cytosine.
(a)Calculate the number of thymine bases in this section.[3]
(b)Calculate the percentage of guanine bases in this section. Give your answer to the nearest whole number.[1]
Show answer
(a) 420
total bases = 2 × 600 = 1200 (B1); C = G = 180, so A + T = 1200 − 360 = 840 (M1); T = 420 (A1).
(b) 15 %
180 ÷ 1200 × 100 = 15 % (B1).
Q30DNA structure & base pairing · Challenge · Paper 4 style · 4 marks
The base composition of DNA from two sources was measured.Sample : A % : T % : C % : G %P : 30 : 30 : 20 : 20Q : 25 : 33 : 20 : 22
(a)Explain why, in double-stranded DNA, the percentage of adenine is always equal to the percentage of thymine.[2]
(b)Suggest which sample, P or Q, comes from a virus that has only a single strand of DNA. Give a reason using the data.[2]
Show answer
(a) Every A on one strand is paired with a T on the other strand (complementary base pairing), so the numbers must be equal.
B1 A pairs with T; B1 one-to-one across the two strands.
(b) Q; its percentage of A (25) is not equal to T (33) and C (20) is not equal to G (22), so the bases are not paired.
B1 Q; B1 A ≠ T / C ≠ G in Q (or A = T and C = G in P).
Similar questions
Questions that test the same subtopic in different years, grouped so you can work on one skill at a time.
References only: find each question in its paper (official link where available, otherwise through your school).
